گروه کیهان‌شناسی

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تحلیل مبتنی بر توپولوژی از میدان‌های کیهانی

11 ژوئن 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

سیدمحمدصادق موحد

دانشکده فیزیک، دانشگاه شهید بهشتی، تهران

گروه کیهان‌شناسی محاسباتی CCG-SBU

آزمایشگاه میان‌رشته‌ای ابن سینا

تحلیل مبتنی بر توپولوژی از میدان‌های کیهانی

چکیده: با ارتقای کمَی و کیفی دستاوردهای رصدی از میدان‌های کیهانی، پرداختن به روشهایی که اساساً بر داده‌ها از حیث تنوع، کمیت و آهنگ تولید، پایه‌گذاری‌شده‌اند، به یکی از موضوع جذاب در تبیین ویژگی‌هایی که با سایر روشهای سنتی پرمایش و مقیدسازی انجام می‌شد، مبدل شده‌است. از سویی دیگر چالشهای متعددی در تعیین مقادیر پارامترهای کیهانی وجود دارد و همچنین دریچه‌ نوینی برای جستجو در کیهان یعنی امواج گرانشی معرفی شده است. در این سخنرانی، با انگیزه بررسی چالشها و همچنین رصد ویژگی‌های عجیب در کیهان، به معرفی خواص موضعی و همچنین خواص سراسری (تعداد مؤلفه‌ها، حفره‌ها) و پایداری‌ آنها نسبت به تغییرات مقدار پارامتر دینامیکی آستانه، می‌پردازم. این ابزار می‌تواند در مقایسه با تابعی‌های مینکوفسکی، خواص اویلر و توپولوژی جینس، اطلاعاتی به دور از واگنی‌های احتمالی فراهم ‌کنند، بررسی می‌کنم. در پایان به این نکته اشاره ‌خواهم کرد که ترکیب این ابزارها به عنوان بردار ویژگی در روش یادگیری ماشین، نیز می‌تواند به غنای دقت آنها بیفزاید. نکات مهم سخنرانی من به قرار زیر خواهد بود:

1) ویژگی‌های توپولوژیک می‌توانند به عنوان ابزاری جدید و البته غیرخطی، به منظور مقیدسازی کمیت‌های کیهانشناختی، بکار روند.

2) روش‌های یادگیری ماشین قادر هستند اهمیت و حساسیت هریک از روش‌های بکارگرفته شده که مبتنی بر معیارهای توپولوژیک و هندسی هستند را تعیین کنند.

یکشنبه 22 خرداد 1401، ساعت 19:00

Sunday 12 June 2022 – 19:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

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Cosmological Collider Physics using Primordial Clocks and Clicks

11 ژوئن 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Reza Ebadi

Department of Physics, University of Maryland

Quantum Technology Center, University of Maryland

Cosmological Collider Physics using Primordial Clocks and Clicks

Abstract: On-shell production of heavy degrees of freedom during the inflationary era can be imprinted on cosmological correlation functions, thereby offering a unique opportunity of cosmological spectroscopy at energy scales far beyond what is achievable with any conceivable terrestrial collider. Such “collider signals”, however, are typically Boltzmann-suppressed above the inflationary Hubble scale, limiting the potential reach of the “cosmological collider.” Using a generic framework, we show that classical features in the inflationary landscape excite unsuppressed quantum modes of heavy fields, which leave observable signatures in primordial non-Gaussianity. In addition to unsuppressed, non-Gaussian collider signals, the analyzed models predict scale-dependence as well as precursor signatures in the inflationary power spectrum. Due to their rich phenomenology, these models present viable targets for next-generation cosmological surveys.

یکشنبه 8 خرداد 1401، ساعت 19:00

Sunday 29 May 2022 – 19:00 Tehran Time

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Probing the dark universe using Optimal Transport theory

21 می 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Farnik Nikakhtar

Department of Physics and Astronomy, University of Pennsylvania

Department of Physics, Yale University

Probing the dark universe using Optimal Transport theory

Abstract: Astronomical observations strongly suggest that the universe is mostly dark. Its two dominant components, dark energy and dark matter, remain among the most mysterious concepts in cosmology today. The effects of these two substances are imprinted in the remaining few percent of the universe that consists of normal (baryonic) matter. Dark energy is responsible for the accelerating expansion of the universe and the existence of dark matter is deduced from the orbital properties of stars in galaxies. In this talk, we discuss the observable effects of both these phenomena and applications of the Optimal Transport (OT) theory to approach them. OT has a fundamental connection to physical problems as most phenomena in nature are governed by optimization principles. Applications of OT touch many fields of physics from quantum mechanics, to general relativity, among others. We address two independent astrophysical problems using OT techniques: Recovering the Galactic Potential with Optimal Transport Theory, and Optimal Transport Reconstruction of Baryon Acoustic Oscillations. Our results would develop novel ways to place stronger constraints on cosmology and dark energy; while also revealing the distribution of dark matter in galaxies, thus constraining dark matter’s properties.

یکشنبه 1 خرداد 1401، ساعت 19:00

Sunday 22 May 2022 – 19:00 Tehran Time

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H0-tension, CMB temperature and Unparticles

15 می 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

H0-tension, CMB temperature and Unparticles

Maryam Aghaei

Department of Physics and Astronomy, Sejong University, South Korea ;

Department of Physics, School of Natural Science,

Ulsan National University of Science and Technology (UNIST), South Korea

Abstract: H0-tension is one of the late-time cosmology challenges for ΛCDM alongside cosmological constant problem, missing satellites and many others. Extensions to ΛCDM have been approached mostly by models with an extra parameter. Here, I introduce a framework for evaluating the viability of models of late-time cosmology by investigating unparticle cosmology as a case study. This model derives from a finite temperature broken conformal symmetry of radiation, described by a non-radiative correction with unknown sign in energy density. This symmetry breaking has a sign ambiguity, in which corrections about the IR fixed point are normal or tachyonic. Focusing on H0-tension for positive sign, the model predicts H(z) in the future to be zero or constant, the latter pointing to a stable de Sitter phase for our Universe. Therefore, in this case, unparticle cosmology fails to ameliorate H0-tension, but it provides a pointer to holographic dark energy. That is while negative sign scenario possesses a late-time temperature Tc≃4T_CMB associated with Ω_U≃1 (Ω_U≃10⁴ Ω_CMB). The CMB is hereby exposed to an enormous heat bath, but the age of the Universe constrained by the globular clusters, independently puts a constraint on any heat exchange from unparticles to the CMB in ΛCDM. Accordingly, we estimate the cross section of unparticles interactions with CMB photons to be σγ_U≲10^-40 m²=10^-3 nb.

Based on:

1- Phys. Rev. D 104, 083511

2- arxiv.org/abs/2203.16076

یکشنبه 25 اردیبهشت 1401، ساعت 15:00

Sunday 15 May 2022 – 15:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

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Testing General Relativity and the gravity constants

8 می 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Vahe Gurzadyan

Center for Cosmology and Astrophysics Alikhanian National Laboratory, Armenia

Testing General Relativity and the gravity constants

Abstract: The unrevealed nature of the dark energy and dark matter outline the importance of high precision testing of the General Relativity and the development of modified gravity theories. The testing of the Lense-Thirring effect predicted by General Relativity by means of satellite experiments will be discussed, including the recently obtained results and the prospects. Also, I’ll discuss the weak-field modification of the General Relativity involving the cosmological constant, which leads to consequences regarding the notion of physical constants, as well as for the description of groups and clusters of galaxies and their large scale motions.

یکشنبه 18 اردیبهشت 1401، ساعت 19:00

Sunday 8 May 2022 – 19:00 Tehran Time

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Ultralate transition of the strength of gravity as a resolution of the Hubble crisis

1 می 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Leandros Perivolaropoulos

Department of Physics, University of Ioannina, Greece

Ultralate transition of the strength of gravity as a resolution of the Hubble crisis: Observational hints and theoretical implications

Abstract: The 5σ mismatch of the measured value of the Hubble constant between observational probes calibrated at early times (sound horizon calibrated at z~1100) and late times (SnIa absolute magnitude calibrated at z<0.01) is currently the most significant challenge for the standard LCDM model. I discuss the potential of a sudden change of the strength of gravity occurring at some specific time during the last 150Myrs (z<0.01) to resolve this tension without affecting the other observational successes of the standard LCDM model. I also review the effects of such a transition on the growth rate of cosmological perturbations and show that the delay of this growth due to such a transition is consistent with indications for such a delayed growth coming from weak lensing and redshift space distortion cosmological observations. Finally, I discuss the consistency of such a transition with other astrophysical data including Tully-Fisher observations, galaxy survey data and solar system chronology. Theoretical models that are consistent with such a transition will also be discussed. Such models include an ultra-late decay of the false vacuum in the context of a scalar tensor modified gravity theory and a generalized symmetron screening mechanism based on an explicit symmetry breaking in addition to the standard spontaneous symmetry breaking mechanism.

یکشنبه 11 اردیبهشت 1401، ساعت 18:00

Sunday 1 May 2022 – 18:00 Tehran Time

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Statistics of the nearest neighbor and the cosmic web

24 آوریل 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Mohammad Ansari

Physics Department, Sharif University of Technology

Statistics of the nearest neighbor and the cosmic web

Abstract: The cosmic web is a rich source of information, which is appropriate to add a constraint on parameters of the standard model of cosmology or to study models beyond the standard model. On large scales, the two-point correlation function is a proper method to extract information. But in non-linear scales, the density field is non-Gaussian, so we need alternative approaches. In this presentation, we discuss these approaches. Specifically, we discuss the nearest neighbor statistics and show that one can study neutrino mass and fuzzy dark matter, using this method. Furthermore, we introduce the angle between nearest neighbors as a new probe for non-linear scales and the cosmic web.

یکشنبه 4 اردیبهشت 1401، ساعت 18:00

Sunday 24 April 2022 – 18:00 Tehran Time

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https://vc.sharif.edu/ch/physics-high-energy

سمینار مشترک گروه فیزیک انرژی بالا و گروه کیهانشناسی

18 آوریل 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Georges Obied

(Harvard University)

Inflation and light Dark Matter constraints from the Swampland

Abstracts: I will explore the interplay between Swampland conjectures and models of inflation and light Dark Matter. To that end, I will first review the weak gravity conjecture (WGC) and the related Festina Lente (FL) bound. These have implications for light darkly and milli-charged particles and can disfavor a large portion of parameter space. The FL bound also implies strong restrictions on the field content of our universe during inflation and presents an opportunity for inflationary model building. At the same time, it rules out some popular models like chromo-natural inflation and gauge-flation. Finally, I will review another Swampland conjecture related to Stückelberg photon masses and discuss its implications for astro-particle physics.

سه‌شنبه ۳۰ فروردین ۱۴۰۱، ساعت ۱۸

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A new picture of the cosmos: a two-sheeted, CPT-symmetric universe

16 آوریل 2022 Mojahed Parsi Mood دیدگاه‌تان را بنویسید:

Latham Boyle

Perimeter Institute for Theoretical Physics

A new picture of the cosmos: a two-sheeted, CPT-symmetric universe

Abstract: After reviewing some key hints and puzzles from the early universe, I will introduce recent work with Neil Turok suggesting a rigid and predictive new approach to addressing them.

Our universe seems to be dominated by radiation at early times, and positive vacuum energy at late times. Taking the symmetry and analyticity properties of such a universe seriously suggests a picture in which spacetime has two sheets, exchanged by a symmetry that, in turn, selects a preferred (CPT-symmetric) vacuum state for the quantum fields that live on the spacetime. In this picture, the Big Bang may be regarded as a kind of mirror.

I will explain how this line of thought suggests new explanations for a number of observed properties of the universe, including: its homogeneity, isotropy and flatness; the arrow of time; several properties of the primordial perturbations; the nature of dark matter (which, in this picture, is a right-handed neutrino, radiated from the Big Bang like Hawking radiation from a black hole); and even the existence of three generations of standard model fermions. I will also mention some observational predictions that will be tested in the coming decade, and some key open questions.

یکشنبه 28 فروردین 1401، ساعت 18:00

Sunday 17 April 2022 – 18:00 Tehran Time

اتاق سمینار مجازی –Virtual Seminar Room

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